1. Field of the Invention
The invention relates to a magnetic resonance imaging apparatus and a method for producing a tomogram of an object under medical examination by applying composite RF signals to the object, thereby causing a nuclear magnetic resonance phenomenon so-called "NMR" phenomenon.
2. Description of the Prior Art
In a conventional computerized NMR tomographic apparatus for medical purposes, the so-called multi-slice method is usually employed to improve diagnostic efficiency per unit time. In this method, magnetization or spinning magnets of a large number of slices are excited by a time-division technique to perform signal acquisition, utilizing the waiting time of the magnetization recovery after excitation or inversion of a single slice. More specifically, a linear magnetic field gradient in the direction perpendicular to a specific slice is applied to an object placed in a uniform static magnetic field, and a resonance frequency is linearly changed along this direction. Under this condition RF (radio frequency) pulses having a bandwidth corresponding to slice thickness with a resonance frequency as a center frequency are sequentially applied to the object to excite and invert magnetization, and to perform echo signal convergence. The actual data acquisition involves the following two methods. To achieve echo signal convergence, magnetization of a specific slice is first inverted and thereafter excited. For the same purpose, the magnetization is excited without magnetization inversion. The magnetization is rotated by 90.degree. with a so-called "selective 90.degree. RF pulse," and is inverted, or converged by a so-called "selective 180.degree. RF pulse."
Such a conventional NMR imaging apparatus is disclosed in U.S. Pat. No. 4,484,138 to Bottomley et al.
The excitation and inversion of the magnetization, and the echo signal convergence act only on a specific region of the object and do not influence the magnetization occurring outside this region at all. This is an ideal condition, not an actual condition. To reduce the adverse influence of the magnetization of the slices adjacent to the specific slice, a comparatively large interval between the adjacent slices must be introduced. If such a broad interval is employed between the adjacent slices, the NMR signal acquisition about these slices located in this interval may fail. In practice, it is difficult for a selective 180.degree. RF pulse to uniformly invert the magnetization in the specific region. The inversion recovery image of the excited specific region can hardly have higher quality.
An object of the invention is to provide a magnetic resonance imaging apparatus and a method by which the interval between an excited slice of an object and an adjacent slice thereof can be reduced as shortly as permitted by using a composite 180.degree. pulse in conjunction with an appropriately controlled field gradient.
Another object of the invention is to provide a magnetic resonance imaging apparatus which can reconstruct a high-quality inversion image of the specific slice of an object by applying a specific composite RF pulse to the object. A still further object of the invention is to provide a magnetic resonance imaging apparatus by which multi-slice inversion images of the specific slices can be produced without a gap between the successive slices.